Acrylonitrile (ACN) is a monomer used extensively in the production of plastics, synthetic fibers, and rubber. In previous assessments conducted by IARC and the EPA, ACN was classified as a probable human carcinogen based on limited evidence in humans and sufficient evidence in laboratory animals. Specifically, EPA had determined that there was an association between ACN exposure and lung cancer based on a study by O'Berg (1980, J. Occup. Med. 22, 245-252). However, a follow-up of this cohort (O'Berg et al., 1985, J. Occup. Med. 27, 835-840) shows no statistically significant excess of lung cancer mortality or incidence. Our evaluation of the more recent human database taken as a whole shows that there is not a clear association between ACN exposure and human cancer, yet the studies have insufficient power to be able to rule out a small increase. In laboratory rats, however, ACN has been shown to be clearly carcinogenic by the oral and inhalation routes. Applying the methodology of EPA's proposed 1996 cancer risk assessment guidelines to the rat tumor data, the estimated upper bound on the excess lifetime risk at continuous exposure to 1 microgram/m3 ACN is calculated to be in the range of 8.2 x 10(-6) to 1.1 x 10(-5).